1. Field of the Invention
The present invention relates to a novel oxopyrazine derivative or a salt thereof, a herbicide containing the same as an active ingredient, and a method of using the same.
2. Description of Related Art
Several compounds among the 1,3-cyclohexanedione derivatives which have been acylated at the 2-position with an arylcarbonyl group, have already been commercially available as agrochemicals. For example, mesotrione is attracting public attention as a foliar applied herbicide for corn having a new mechanism of action. A 1,3-cyclohexanedione is a tautomer, which exists also as 1-hydroxycyclohexene-3-one which is an enol form thereof, and this derivative has been developed as a compound for various agrochemical uses.
For example, a derivative having the aryl group of the arylcarbonyl group substituting at the 2-position changed to various heteroaryl or cycloalkyl groups such as pyrazine (see, Patent Document 1), a derivative having the 1,3-cyclohexanedione ring fused at the 4- and 5-positions with a cyclopropane ring (see, Patent Document 2), a derivative having the arylcarbonyl group at the 2-position changed to a pyrimidin-5-ylcarbonyl group derivative (see, Patent Document 3), a derivative having the arylcarbonyl group at the 2-position changed to a pyrazin-2-ylcarbonyl group derivative (see, Patent Document 4), a derivative having the arylcarbonyl group at the 2-position changed to a 1,2,3-thiadiazol-5-ylcarbonyl group derivative (see, Patent Document 5), derivatives having the arylcarbonyl group at the 2-position changed to a pyridinecarbonyl group derivative (see, Patent Documents 2, 6, 7, 8, 9 and 10), derivatives having the arylcarbonyl group at the 2-position changed to a quinolinecarbonyl group derivative (see, Patent Documents 11 and 12), a derivative having the arylcarbonyl group at the 2-position changed to a heteroarylcarbonyl group formed from a benzazole (see, Patent Document 13), a derivative having the arylcarbonyl group at the 2-position changed to an azolecarbonyl group derivative formed from an 1,2-azole (see, Patent Document 14), and the like have been reported. Furthermore, derivatives in which the 4-position and the 6-position of the 1,3-cyclohexanedione ring are crosslinked by an alkylene group such as an ethylene group, have also been reported (see, Patent Documents 8, 11, 12, 13, 14, 15 and 16). In addition, it has also been reported to have a 3,5-cyclohexanedione-1-thione ring in which the 5-position of the 1,3-cyclohexanedione ring has been changed to an oxo group, and the 1-position has been changed to a thiocarbonyl group (see, Patent Document 17).
As such, a large number of cyclohexanedione compounds having herbicidal activity have been reported, but there is no known cyclohexanedione compound having a dihydropyrazine ring substituted with an oxo group or a thioxo group (in the present specification, these may be generically called (thio)oxo), such as the compound of the present invention represented by the following formula [I].    Patent Document 1: EP-283261    Patent Document 2: WO 91/00260    Patent Document 3: U.S. Pat. No. 4,708,732    Patent Document 4: DE-3902818    Patent Document 5: EP-338525    Patent Document 6: JP-A No. 2-78662    Patent Document 7: JP-A No. 3-52862    Patent Document 8: JP-A No. 4-29973    Patent Document 9: WO 96/14285    Patent Document 10: WO 2000/39094    Patent Document 11: JP-A No. 2000-16982    Patent Document 12: WO 2000/14069    Patent Document 13: WO 2000/68210    Patent Document 14: JP-A No. 2005-200401    Patent Document 15: WO 2005/058831    Patent Document 16: WO 2006/066871    Patent Document 17: DE 10256354